Collect a birth year instead of a full date of birth

Signup and the profile page now ask for a four-digit year between 1900 and
the current year. Anything else — a short year, a full date, a year in the
future — is discarded rather than stored, so a typo cannot leave a nonsense
age on the record.

The year lives in a new us_birth_year user meta rather than reusing
us_date_of_birth, which would have left one key holding two formats. The old
key is not migrated in bulk. Instead GuardianService handles it in two
halves: birthYear() falls back to the year of the old date when the new key
is absent, so a student added before this change still shows one, and
setBirthYear() deletes the old date on every save.

That deletion is what makes the fallback safe rather than merely tidy.
Without it, clearing the birth year on a student who predates the change
would leave the old date behind for the fallback to read straight back, and
the year could never be cleared at all.

Stored in user meta, so no Schema.php change and no USC_VERSION bump.

Closes #147

Co-Authored-By: Claude Opus 5 <[email protected]>

src/Auth/RegistrationPage.php

@@ -477,11 +477,11 @@ class RegistrationPage {
 
 	/**
 	 * The child blocks submitted with a guardian signup, as
-	 * `children[<n>][name|dob|answers]`. Blocks with no name are dropped rather
+	 * `children[<n>][name|birth_year|answers]`. Blocks with no name are dropped rather
 	 * than rejected — the form always renders one spare block for "add another",
 	 * and an untouched spare is not a mistake the guardian needs telling about.
 	 *
-	 * @return list<array{name: string, dob: string, answers: array<int, string>}>
+	 * @return list<array{name: string, birth_year: string, answers: array<int, string>}>
 	 */
 	private function submittedChildren(): array {
 		// The submit nonce is verified by the caller before this runs.
@@ -508,9 +508,9 @@ class RegistrationPage {
 			}
 
 			$out[] = [
-				'name'    => $name,
-				'dob'     => sanitize_text_field( Val::string( wp_unslash( $child['dob'] ?? '' ) ) ),
-				'answers' => $answers,
+				'name'       => $name,
+				'birth_year' => sanitize_text_field( Val::string( wp_unslash( $child['birth_year'] ?? '' ) ) ),
+				'answers'    => $answers,
 			];
 		}
 
@@ -528,7 +528,7 @@ class RegistrationPage {
 	 * re-register and children they never confirmed, so it is undone entirely and
 	 * they simply try again.
 	 *
-	 * @param list<array{name: string, dob: string, answers: array<int, string>}>                  $children
+	 * @param list<array{name: string, birth_year: string, answers: array<int, string>}>           $children
 	 * @param list<Question>                                                                       $questions
 	 * @param list<array{policy: Policy, version: \Unsupervised\Schedular\Policy\PolicyVersion}>   $policyForms
 	 */
@@ -536,7 +536,7 @@ class RegistrationPage {
 		$created = [];
 
 		foreach ( $children as $child ) {
-			$childId = $this->guardians->createChild( $guardianId, $child['name'], $child['dob'] );
+			$childId = $this->guardians->createChild( $guardianId, $child['name'], $child['birth_year'] );
 
 			if ( $childId instanceof \WP_Error ) {
 				foreach ( $created as $id ) {